1. Field of the Invention
This invention relates to light emitters, and more particularly, to nitride-semiconductor based light emitting devices with high power, high luminous efficiency and optical apparatus using the same.
2. Description of the Related Art
Nitride based semiconductor materials have drawn tremendous attention during last decade due to its enormous potential to be utilized in white lighting and high-power semiconductor devices. In a nitride semiconductor light emitting device, InGaN is often used for a quantum well (QW) layer included in its light emitting layer and it emits a wide range of colors from blue to orange just by adjusting the In content in InGaN QW. In recent years, blue, green and violet semiconductor light emitting diodes (LED) and lasers have been developed utilizing the characteristics of the nitride semiconductor light emitting devices.
For light generation in semiconductor lasers and LEDs, radiative recombination of electrons and holes in an active layer is used. The active layer can be a usual p-n junction, hetero-junction based on single quantum well or multiple quantum wells. For fabrication of a highly efficient device, the number of carriers recombined inside the active layer should be maximized and the number of carriers recombined, leaked or overflown outside the active layer should be minimized. This needs optimization of capture rates for electrons and holes into the active layer. Less confinement of electrons and holes would result in some of the electrons and holes not captured in the active layer to escape the active layer and recombine outside it. This results in low efficiency of these devices.
Moreover, the high temperature performance is keenly related to the amount of carriers, such as electrons and holes, confined in the active region. The threshold current (Ith) is the minimum current required to establish the population inversion necessary for lasing, and can be expressed by the following temperature-dependent formula:Ith(T)=Ith(TR)e(T−TR)/To,  [1]where To is the characteristic temperature, Ith(TR) is the threshold current at reference temperature and T is the operating temperature. The higher the value of To, less is the temperature dependence of the threshold current, higher is the stability of the laser during oscillation especially at high output power.
Typically, To is in the range of about 160-170° K in the temperature range of 20° C. to 50° C. for a 1.5 mm-long InGaAs—AlGaAs single quantum-well (SQW) laser emitting at 980 nm (Weidmann et al. Journal of Quantum Electronics No. 1 Vol 38 2002). To is supposed to be constant—at least within a certain temperature range—but depends on the effective cavity length of the laser. A corresponding relationship can be found for the temperature dependence of the differential quantum efficiency as a second characteristic temperature T1. The differential quantum efficiency is linked to the mirror loss, the internal loss, and the internal quantum efficiency (all of which are assumed to be temperature-dependent, except for the mirror loss) by the equation. The temperature dependence of the mirror loss is neglected since the thermal expansion of the cavity is very small. In general, T1 is a few times larger than T0, and is about 560° K for the above-mentioned type of lasers.
The degradation of the electro-optical characteristics of quantum well lasers with increasing temperature is mainly attributed to thermionic emission and overflow of the carriers out of the quantum well (Ziel et al. Applied Physics Letters, vol 58, pp. 1437-1439, 1991). Because of the exponential dependence of the thermionic emission time on the barrier potential height of the quantum well (Jandelet et al. Proc. SPIE, vol 3626, pp. 217-229, 1999), increasing the barrier height seems to be the most effective way to increase the carrier confinement and, therefore, to improve high-temperature operation. The electrons are usually of major interest because of their much smaller effective mass and density of states in the quantum well.
One of the ways to increase the barrier height is to introduce high-band gap layers as quantum-well electron barriers. For InGaAs QWs on GaAs substrate, this is done by increasing the aluminum content of a typical AlGaAs barrier. However, this leads to a lower epitaxial quality and a higher thermal resistance for aluminum content up to 50%. Furthermore, to avoid a higher voltage drop and to keep the optical confinement factor constant, the aluminum content of the cladding layers also has to be raised, which further degrades the thermal and epitaxial quality. Superlattice barriers can also be used to realize large quantum well potential barriers, simultaneously having low average aluminum content. However, in an undoped state, they increase the series resistance and generate an additional voltage drop due to their limited electronic transport properties (Weidmann et al. Journal of Quantum Electronics No. 1 Vol 38 2002).
Since high optical output power demands minimized thermal and electrical resistance, increasing the quantum-well barrier height do not seem to be quite viable.
An alternative way to increase the electron confinement is to use carrier blocking layers (U.S. Pat. No. 5,764,668).
An Electron blocking layer, typically, is a layer inserted between the light emitting layer and the p-type waveguide layer (see FIG. 1). It has a wider band gap than the barrier material of the active region, with the purpose of preventing injected electrons from overflowing the active region. Similarly, hole blocking layer is a layer inserted between the light emitting layer and the n-type waveguide layer of the device. It has a wider band gap than the barrier material of the active region, with the purpose of preventing injected holes from overflowing the active region.
For GaAsSb based material system AlGaInP confinement layers have been suggested (U.S. Pat. No. 6,931,044) as the carrier blocking layers. Weidmann et al. have also demonstrated band-edge aligned quarternary AlGaInP as hole barrier and AlGaAsSb as electron barrier for an InGaAs QW on GaAs substrate (Journal of Quantum Electronics No. 1 Vol. 38, pp. 67-72 2002). The characteristic temperature of their laser structure was improved by about 50 to 60° K relative to a similar laser structure without the blocking layer. Additionally, the introduction of the blocking layer did not lead to any additional voltage drop or series resistance. The higher temperature stability of threshold current is mainly attributed to the suppression of carrier leakage and reduced free-carrier absorption at elevated temperatures by preventing thermionic emission and carrier overflow at elevated operating temperatures.
In GaN based material system meant for LEDs and lasers, either AlGaN or AlInGaN is routinely used as electron blocking layer (Piprek et al. Journal of Quantum Electronics No. 9 Vol 38 2002). The job of an electron blocking layer can be more vividly explained through the schematic diagram of FIG. 1. Electrons in the conduction band, under bias, are injected from n-type clad and waveguide. Drifting through n-type region they eventually fall into the quantum wells. Similarly, holes in the valence band, under bias, are injected from p-type clad and waveguide. Drifting through p-type waveguide layer they eventually rise in to the quantum wells. Electrons and holes radiatively recombine at the well to emit photons with a wavelength determined by the band-gap of the well layer. All the electrons and holes are supposed to recombine in the QWs. However, due to lighter mass and host of other reasons such as spontaneous and piezo-electric field in the well region, electrons tend to overflow the quantum wells and thus recombine with holes non-radiatively outside the wells. So the primary objective of the electron-blocking layer is to stop electrons from overflowing over the QW.
However, this electron blocking layer (AlGaN or AlInGaN) typically is not band-edge aligned to the barrier layer or waveguide layer. So it not only does block electrons in the conduction band, from overflowing to the p-side but also tends to block holes in the valence band from reaching the QW (See FIG. 1). In principle, an ideal carrier blocking layer is the one which blocks one type of carriers, either electrons or holes, and does not block the other.
The present invention is based on choosing right materials with precise compositions so that they are supposed to behave like ideal carrier blocker. FIG. 2 conceptually shows a schematic of a QW structure with band-aligned electron blocking layer in the n-side, which not only does block electrons in the conduction band but at the same time ‘does not’ block holes in the valence band. Similarly, FIG. 3 conceptually shows a schematic of a QW structure with band-aligned hole blocking layer in the p-side, which not only does block holes in the valence band but at the same time ‘does not’ block electrons in the conduction band. Additionally, FIG. 4 shows a schematic of a QW structure with both band-aligned electron blocking layer in the p-side, which is transparent to holes in the valence band and band-aligned hole blocking layer in the n-side, which is transparent to electrons in the conduction band.